Maclean served as Liberal MP for Bath 1906-10, Peebles and Selkirk 1910-18, Peebles and Southern Midlothian 1918-22 and North Cornwall 1929-32. When Asquith lost his seat at the 1918 general election, Maclean was thrust into the leadership of the Liberal Parliamentary Party while Asquith retained formal leadership of the party overall. His leadership was an unexpected success, assaulting the Coalition’s Liberal pretentions and resuscitating party organization in the country. He achieved cabinet office in the last year of his life when he became President of the Board of Education in 1931 in the National government. His son was Donald Duart Maclean the spy who worked with Guy Burgess and Kim Philby.